1. Field of the Invention
The present invention relates generally to electro-optic modulators with acoustic damping and more particularly to electro-optic modulators with acoustic damping which utilize an acoustic coupler to link the modulator crystal to an acoustic damper wherein the acoustic coupler and the acoustic damper are shaped to insure acoustic reflection and scattering within the acoustic damper.
2. Description of the Prior Art
In modern laser apparatus, the laser cavity emission is controlled by regulating the voltage applied to an electro-optic modulator, such as a LiNbO.sub.3 crystal. The voltage change across the electro-optic crystal will cause the power within the laser cavity to either be emitted or accumulated depending upon the laser's design due to the polarization change of the light traveling through the crystal. Additionally, the voltage change across the electro-optic crystal generates an undesirable acoustic wave which also alters the polarization of the light traveling through the crystal.
In a typical electro-optic modulator, the acoustic wave, generated by the voltage change, continues to reverberate within the electro-optic crystal even after the voltage applied to the crystal reaches its desired value. This continued reverberation modulates the polarization of the light, thus perturbing the desired laser response. This perturbation may be in the form of light leakage during a period of power buildup prior to emitting a pulse or it may alter the amount of radiation leaving the laser cavity. The acoustic waves also limit the pulse rate at which the laser can emit uniform pulses and distort the temporal profile of the laser pulse.
A typical method of damping acoustic waves in an electro-optic modulator is to immerse the modulator crystal in an appropriate liquid to dampen the acoustic waves. The liquid enveloping the crystal is chosen to match the acoustic impedance of the modulator crystal. This method suffers, however, from several deficiencies including the potential for leakage of the fluid or vaporization or decomposition of the fluid upon the occurrence of a laser pulse with sufficiently high energy. The tendency of liquid to not support shear waves, a property desirable in order to damp acoustic waves, further limits the use of liquid in damping acoustic waves.
An alternative apparatus for damping acoustic waves is the electro-optic device disclosed by Kiefer, et al. in U.S. Pat. No. 3,653,743 which has an acoustic energy absorbing material bonded thereto. However, the device is limited to utilizing dampers which have their planar faces parallel to the electro-optic crystal's face and which have cross-sectional dimensions which are substantially equivalent to those of the crystal in order to maintain best operation. Thus, while this device does couple acoustic waves from the electro-optic crystal and dampen those waves, these limitations cause the device to be inadequate in several respects. The damper's geometry is not designed to prevent reflections from the damper's edge from returning to the crystal following only one pass through the damper and disturbing laser modulation upon their reentry into the electro-optic crystal. Furthermore, when the dampers couple acoustic waves at a resonant frequency, the acoustic energy is stored in the dampers as a standing wave which requires a lengthy time in which to dissipate.
It would be desirable to develop a electro-optic modulator with acoustic damping whose geometry is such that a plurality of reflections will occur within the acoustic damper to scatter the acoustic wavefront before the acoustic energy is reflected towards the crystal. Furthermore, it would be desirable for the electro-optic crystal to be shaped so that acoustic energy traveling within the crystal in a direction where there is no acoustic damper attached is directed towards an interface where there is an acoustic damper attached.